1. Field of the Invention
This invention relates generally to eFuses (electrically programmable fuses). More specifically, this invention relates to sensing whether an eFuse is blown or not blown.
2. Description of the Related Art
Electronic systems often require function to be enabled, disabled, or modified after semiconductor chips used in the electronic systems have been manufactured. For example, a common chip may be designed to serve multiple applications, the chip originally having circuitry to support all of the multiple applications. After manufacture of the chip, eFuses can be blown to personalize the chip for a particular specific application. For a second example, sometimes chips are manufactured imperfectly and portions of the chip are unusable. A computer processor chip may be designed to have a 128 KB (kilobyte) cache, but testing may determine that only 64 KB of the 128 KB is functional. If the remainder of the chip is functional, the chip may still be used, but information must be stored on the chip so that no attempt to use the nonfunctional 64 KB portion of the 128 KB cache is performed.
On modern semiconductor chips eFuses are often used to store such information. An eFuse is electronically programmable, and may be programmed by blowing the eFuse after a chip is manufactured. In many applications, the eFuse is blown even after an electronic system utilizing the chip has been in operation for some time.
An eFuse comprises a silicided polysilicon conductor. Silicide has been widely used in semiconductor products to reduce resistance of a polysilicon conductor, for example polysilicon gates used in Field Effect Transistors (FETs), or a doped silicon region, such as a source or drain of a FET. An eFuse is blown by directing a current of sufficient magnitude and duration through the eFuse to remove by melting or electromigration at least a portion of the silicide between a first end and a second end of the eFuse. Removal of at least a portion of the silicide changes an electrical resistance between the first end and the second end of the eFuse. Descriptions of eFuses can be found in U.S. Pat. No. 6,368,902, “Enhanced eFuses by the local degradation of the fuse link”, by Chandrasekharan Kothandaraman, et al, and U.S. Pat. No. 6,624,499, “System for programming fuse structure by electromigration of silicide enhanced by creating temperature gradient”, by Chandrasekharan Kothandaraman, et al.
Polysilicon has a relatively wide tolerance on resistance. Silicided polysilicon, while having a lower resistance, also has a relatively wide tolerance on resistance. Furthermore, there is a significant range in exactly how much silicide is removed from polysilicon on an eFuse when the eFuse is blown. Beyond normal process variations and variations on how much silicide is removed, resistance of silicided polysilicon and unsilicided polysilicon varies with temperature. These variations have, in previous electronic systems having eFuses, made the task of sensing whether an eFuse is blown or unblown difficult and problematic.
Therefore, there is a need for a method and apparatus that provides for reliable, simple, and fast sensing of an eFuse to determine if an eFuse is blown or unblown.